Choosing the "right" solar thermal system

[DesMoines]

I'm renovating a house in the Seattle area. Part of the renovation includes changing the roof lines for good solar exposure. Heating will be hydronic radiant upper floor (1900 ft2) and hydronic wall panels in finished daylight basement (1400 ft2). Floors will be connected with an open staircase. Air sealing and high levels of insulation are being incorporated. Being on Puget Sound, we're sacrificing insulated west walls for an expansive view of the Sound using a lot of Serious windows. Very efficient windows but windows none the less. On the rare Western Washington sunny winter afternoon, they'll help in heating the house.

Energy system involves the following:
PV (2 arrays)
Solar thermal (liquid)
ERV
Daikin Altherma air-water heat pump with Daikin DHW tank.

I've been presented an extensive energy package proposal. The solar thermal system proposed is a closed loop pressurized propylene glycol system. It comprises (8) Buderus SKS 4.0 collectors and a 200 gallon PL750/2S Buderus storage tank. This integrates with the Daikin Altherma system with the Daikin hot water tank. After doing some reading and calculating the relatively small amount of DHW I might use per day (sometimes 15-20 over a couple weeks), I broached the subject of a heat dump. It took some coaxing but it was finally agreed that a heat dump system was required. The number of panels may be reduced as well. After doing more reading and research, I really like the idea of a closed loop drainback system. With care taken for proper drainage, overtemp, freezing, and power failure problems are resolved.

The company I'm working with is relatively new and comprised of 3 persons from different backgrounds. Radiant system installer and controls engineer, electrician with extensive PV experience, and a structural engineer. They're good to work with and are open to other ideas. Some of the engineering is being done by a distributor for Buderus so the proposal is probably skewed. I think they're open to building the best system possible and that's what I'm after.

With all this said, I need help in selecting the best type of collector (plate or tube) and if a drainback system is better than a pressurized glycol system. Any other advice is appreciated as well.
Thanks to all in advance.

[Mike90250]

That's some project. Sorry I dont know a lot about the thermal systems yet.

[russ]

I expect the amount of heat generated by the solar thermal panels in the winter time will be minimal - but the Daikin Altherma system is first class.

Evacuated tube type are generally a little bit better in cloudy conditions but then a little bit more than not much is still not much. If you use the solar heated water to preheat the water to the Altherma you maximize the use I expect.

I would most likely go with flat plate due to cost and my belief that the benefit of evacuated tubes under your conditions (Puget Sound - Seattle) are marginal.

A drain back system is nice - full protection from freezing and no worry about the propylene glycol breaking down if overheated.

What are you considering for a heat dump?

The ERV/HRV is a great deal. You need to seal the shell of the house as best is possible and the energy recovery unit will assist greatly with ventilation of the house as well as energy saving.

Window frames - PVC or wood? I would suggest argon gas fill for the windows - krypton is better but here it was too expensive.

[DesMoines]

Hi Russ. Thanks for the reply.

- Yes, we would be using solar thermal storage to heat DHW and heating system water.
- Windows are fiberglass, argon-filled, with suspended film in the argon space between panes. I'll be looking for feedback on how these types of windows perform. Specs are incredible.

If we end up with a drainback system, I won't be needing a heat dump. If for some reason we end up with a pressurized glycol system, a fan coil unit would probably be installed but I sure like the passive dissipator from Apricus. I found that at Naptown's site. Instructions say for use only with Apricus collectors but it would seem you could integrate them with other collectors.

I appreciate the information.
Greg

[DesMoines]

Would love to get more input

Thanks to Mike and Russ for feedback. I had hoped for more discussion and ideas. If anyone else wants to put in their two cents, I'd like to hear it. I'm going to post on at least one other site as I continue to troll for experience. If the complexity warrants paying someone to consult (already have done that), so be it. Anyone?

[russ]

Between the glass panes the low E coating is adhered to the inside of the outer pane. I have Al frames with low E coating and argon fill - love them. Would like the fiberglass but my wife like Al. Winters here are mild so it not a big deal.

Some of the self cleaning glass looks interesting.

I have an air to water heat pump for the hydronic floor heating plus fan coils for heating/cooling. In three years I have yet to turn on the AC. If I did it again I would look at a standard heat pump with coils in each room. Easier to direct the heat where you want it more quickly.

My home is concrete frame and floors with brick fill. With hydronic in floor heating you more or less have to turn it on and leave it on until spring - heating up the mass is slow. Maybe it is a bit more flexible with a wooden house.

Consider solar thermal air heating - it is the most efficient of all solar due to low temperatures involved.

In a home with a high mass such as I have external insulation is really important. İn the summer time the more heat I can keep out of the structure the better.

Will type more as I think of points.
Russ

[peakbagger]

Overheating glycol is not really an issue with flat plate collectors unless you stop the flow through them. As long as there is flow, they dont get much warmer than 80 deg F over the outside temperature. So even on a 100 degree day, they will no get much more than 180 deg F. Glycol is rated for much higher. On the other hand evacuated tubes can get much hotter and you do really need a heat dump that is automated (preferably its should be "fail open" so that if the power goes out the dump is on line. Even with a failed pump, the system may thermosyphon enough to keep the temps down low as long as there is a heat dump, although it should be piped correctly in order for this to happen. Some evacuated tube systems reportedly have a heat dump mounted on the collector rail. There is a lot to be said for a DC pump and a solar collector to run the circulation but many folks like AC pumps as they are less expensive up front.

[russ]

Any place subject to a power failure can easily overheat a flat plate panel. I have that problem here

I have read about many problems with the DC solar thermal pumps - they are typically designed with little safety margin.

[peakbagger]

An AC pump can be misapplied or be of low quality just as easily as a DC pump can be. I think DC pumps cost more as there is less demand for them and there are a lot of low end DC pumps sized for intermittent use that get specified due to pricing purposes for continous use.

The power outage issue is why I like DC with a solar panel. As long as the sun is shining, the system is circulating. Guy Marston in Maine(Artech?) makes DC controllers that are driven by the same solar panel. Knock on wood, I have over 5 years on my original DC pump. Inherently its variable speed, the more light on the PV panel, the faster the pump runs which corresponds to the solar gain on the hot water panel. I do have a spare pump sitting in box just in case.

I am aware that a lot of DC pumps get underspecified when supplied as kits. My kit had a reasonably sized pump, but the company sized the panel for Florida (not Maine). I switched out the original panel with a spare higher wattage panel I had sitting around and it was fine from then on.

Like many other items in SHW system, its a matter of picking the right stuff that will do the work and having the experience to know what the right stuff is.

[preciseenergy]

Right solar system

That is actually very important because the type of solar system you are installing is very important.